The present invention generally relates to molding of plastic containers from preforms. More particularly, the invention relates to the forming of a plastic container from a preform utilizing a hydraulic blow molding process.
Plastic containers are commonly used for the packaging of various products, such as liquid products (including viscous liquids products). One common type of plastic container is the blow molded plastic container, which is often formed of a polyester material, and more specifically, polyethylene terephthalate (PET). Blow molded plastic containers are typically formed by placing a heated preform into a blow mold and then blowing and inflating the preform with air until the preform contacts the interior surfaces of the mold cavity, which define the final shape of the desired container. Once the inflated preform has been held against the interior surfaces of the mold cavity by the pressure of the blow air for a length of time sufficient to “freeze” the plastic, the blow molded container is removed from the mold.
In traditional blow molding, after the heated preform has been introduced into the mold cavity, a stretch rod is often advanced within the preform to initiate longitudinal stretching of the preform before any significant radial expansion of the preform is undertaken by the introduction of blow air. The stretch rod will may remain within the preform during radial expansion and is retracted prior to removal of the resultant container from the molding machine.
The blow molded containers are then transported to the location where the containers will be filled with the intended product. This may include the packaging and shipping of the containers to a remote location or may involve the transfer of the containers to a local facility, where these final steps occur before being shipped to a retailer or end-user.
With the above method, blow molding and filling are distinct and separate steps in the process of producing a product filled container. A newer process involves the use of the actual end product in the molding of the container. Instead of utilizing air as a blowing medium, this new process utilizes a liquid end product, the product being packaged in the container and sold to the end consumer, as the container's molding medium. As used herein, this type of molding is referred to as hydraulic blow molding. In hydraulic blow molding, instead of air, the molding medium is the liquid that is intended to be packaged in the container.
Conventional blow molding, which uses air as a molding medium, utilizes a higher molding medium temperature than hydraulic blow molding. Generally, the temperature of the molding medium used in hydraulic blow molding is in the range of only 10° C. to 100° C. Additionally, because of the liquid nature of the molding medium used in hydraulic blow molding, heat is withdrawn preform at a rate that is faster than when air is the molding medium. Thus, problems may arise in connection with the axial stretching and radial expansion of the preform.
If the molding medium of hydraulic blow molding contacts the material of the preform prematurely, the molding medium can cause localized cooling of the material. This premature cooling can result in portions of the material insufficiently stretching and expanding and, therefore, insufficient and/or improper formation of the resulting container. Also, localized cooling of the material can cause the preform to blow-out, in the area of such cooling, during the molding of the container.
Is it therefore important to minimize the extent to which the molding medium prematurely contacts the plastic of the preform. In other words, it is important to minimize the length of time that the product is in contact with the material of the preform before the preform has been fully axially stretched and radially expanded into the final configuration of the container.
During hydraulic blow molding, a chamber or bore of the nozzle is filled with molding medium that has been delivered from a source at a predetermined system pressure. Once filled with the molding medium, an inlet valve closes thereby constraining the molding medium within the system. The system pressure of the molding medium at this point in in the molding process is minimal, generally in the range of 1 to 2 bar. Next, the outlet of the nozzle is opened, the molding medium is pressurized so as to rapidly increase the pressure within the system and eject the molding medium from the nozzle's outlet into the preform, which is located within a mold assembly associated with the system. However, the raising of the pressure in the system is not instantaneous. The initial pressure driving the molding medium may not be sufficient to ensure that the molding medium does not prematurely contact, or contact for too long a length of time, the material of the preform. If such contact does occur, the result may be the blowing out of the preform or an improperly formed container.
If too much pressure is initially generated within the nozzle before the nozzle's outlet is opened, it is possible that some of the molding medium may breach the seal and drip into the preform. This too may result in a localized cooling of part of the preform and in an improperly formed container.